Method and device for measuring the proportion of an ATM cell stream satisfying a determined criterion

ABSTRACT

A process and device for measuring the proportion of ATM cells satisfying a criterion. The process includes allocating a first numerical value to the cells passing through a specified point of an ATM network for satisfying criterion and allocating a second numerical value to the cells which do not satisfy the criterion. An exponential mean is then performed of the string of values thus allocated to each cell.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a measurement process and deviceapplicable to packet-switched data transmission networks, and inparticular to those known by the abbreviation ATM “asynchronous transfermode”.

2. Discussion of the Background

The packets transported in ATM networks are referred to as cells and itmay be beneficial to know, at a point of a stream of cells, what is, ateach instant, the proportion of cells which satisfy a specifiedcriterion: for example, what is the proportion of cells corresponding toa given connection, to a given service, etc.

If the total rate of the stream of cells is known, as is for example thecase when dealing with the stream of ATM cells travelling along ahighway with a known bit rate, a measurement of proportion makes itpossible to deduce a measurement of bit rate in respect of the cellssatisfying the relevant criterion.

In actual essence, such a measurement should give a result at anyinstant but it should take account of events which have happened in amore or less recent past. A simple sliding average, such as described inpatent application WO-A-94/11972, may suffice in numerous cases but theimplementation of such an algorithm generally calls upon a circularqueue of the latest values, this requiring capabilities for storage andfor indexed access to the memory. Moreover, the horizon of this average,that is to say its capacity to store the past depends directly on thephysical memory available.

SUMMARY OF THE INVENTION

The purpose of the invention is to alleviate the aforesaid drawbacks byvirtue of a process and a device which make it possible to perform ameasurement of proportion requiring minimal hardware resources.

To this end, the subject of the invention is a process for measuring theproportion of ATM cells satisfying a criterion at a specified point of acell-based data transmission network, characterized in that it consistsin allocating a first numerical value to the cells passing through thespecified point which fulfil the criterion and a second value to thecells which do not satisfy the criterion and in performing anexponential mean of the string of values u_(n) thus allocated to eachcell.

The subject of the invention is also a device for implementing theaforesaid process.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will emerge in thedescription which follows with regard to the appended drawings whichrepresent:

FIGS. 1 and 2, devices for implementing the process according to theinvention.

FIGS. 3a to 3 c, examples of the results obtained by applying theprocess according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In its implementation the process according to the invention uses theknown process for calculating the exponential mean and which is noneother than the result of a discrete-time first-order linear filteringperformed on a string of input values u_(n). The string x_(n) of statesof the corresponding filter is defined by the equation:

x _(n+1) =a·x _(n)+(1−a)·u _(n)  (1)

where a is a parameter less than 1, close to 1, which defines thehorizon of the filter. The string x_(n) thus represents a sort of moreor less long-term instantaneous mean of the inputs u_(n). The processaccording to the invention consists in allocating a first value to u_(n)when a cell passing at a specified measurement point of an ATM networkat an instant n satisfies a specified criterion and by fixing a secondvalue for it when the relevant cell does not satisfy this criterion,then in performing a filtering of the values u_(n) according to equation(1) so as to obtain at any instant a mean value x_(n) representing theproportion of the number of cells which satisfy the criterion and whichtravel past the measurement point in the network.

By way of example, by fixing the first value of u_(n) at 256 and thesecond value of u_(n) at zero and by choosing a notable value for a,such as {fraction (15/16)} so as to limit the multiplication anddivision operations to binary shifts, the value of x_(n) then gives thesought-after proportion on a scale from 0 to 241.

A corresponding device for implementing the process according to theinvention is represented in FIG. 1. This comprises a circuit 1 fordetecting cells corresponding to the relevant criterion coupled to anexponential mean calculating circuit 2 represented inside a closeddashed line. The detection circuit 1 supplies the value u_(n)corresponding to the relevant criterion to the input of the calculatingcircuit 2 and which is read for example from a predefined field of theATM cell. The calculating circuit 2 comprises an addition operator 3coupled on a first operand input labelled “+” to a first output of thedetection circuit 1. It also comprises a first shift operator 4 which iscoupled to the output of the addition operator 3 so as to supply thestring of values x_(n). The output of the first shift operator 4 iscoupled to a second operand input labelled “+” of the addition operator3 by way of a second shift operator 5 and of a delay-by-one-bit circuit6 which are linked in series. A third operand input labelled “−” of theaddition operator 3 is coupled to the output of the delay circuit 6. Tocorrespond to the example cited in which a={fraction (15/16)} and u_(n)is fixed at 256 or zero, the first shift operator 4 is adjusted toperform a rightward shift by 4 bits, the second operator 5 is adjustedto perform a leftward shift by 4 bits.

Naturally, the same result can be obtained with the aid of amicroprogrammed microprocessor. In this case, relation (1) becomes:

x _(n+1)=((x _(n)<<4)−x _(n) +u _(n))>>4  (2)

where the operators << and >> are the logical shift operators as in theC language.

Comparable results can also be obtained with other notable values of a,such as (2^(r)−2^(s))/2^(r). For this, it is sufficient to modify thedevice in the manner represented in FIG. 2 in which the elements akin tothose of FIG. 1 are represented with the same references. In this casethe first operator 4 is adjusted to perform a rightward shift by r bitsand the second operator 5 is adjusted to perform a leftward shift by rbits. The device differs from that of FIG. 1 by the introduction of athird shift operator 7 which must be inserted between the delay device 6and the addition operator 3 and of a fourth shift operator 8 between theoutput of the detector 1 and the first operand input of the additionoperator 3, both of them so as to perform a leftward shift by s bits. Inthe case in which a is equal to (2^(r)−2^(r))/2^(r) and for u_(r)=2^(q)or 0 depending on whether the relevant cell does or does not fulfil thecriterion in question, the proportion calculated by the followingequation lies between 0 and 2^(q)−2^(r−s)+1, i.e.:

x_(n+1)=(x _(n) <<r−x _(n) <<s+u _(n) <<s)>>r  (3).

Examples of results obtained by implementing the process according tothe invention are shown in FIGS. 3a, 3 b and 3 c.

The curve represented in FIG. 3a shows the proportion measured when onecell out of two satisfies the given criterion.

The curve represented in FIG. 3b shows the proportion measured when eachcell satisfies the given criterion with a probability of 60%. (Chancesimulation by a uniform random variable).

The curve represented in FIG. 3c shows the proportion measured when thecell rate satisfying the given criterion is fitted to the thin-linesinusoid. (If the cell at the instant n satisfies the criterion, thenext cell satisfying the criterion will arrive at the instant p, theinteger part of n+1/r(n)).

What is claimed is:
 1. Process for measuring the proportion of ATM cellssatisfying a criterion at a specified point of a cell-based datatransmission network, characterized in that it consists in allocating(1) a first numerical value to the cells passing through the specifiedpoint which fulfil the criterion and a second value to the cells whichdo not satisfy the criterion and in performing an exponential mean (2)of the string of values u_(n) thus allocated to each cell.
 2. Processaccording to claim 1, characterized in that the exponential mean (2) ofthe string of values u_(n) is performed with the aid of a first-orderdigital filter defined by the equation: x _(n+1) =a x _(n)+(1−a)u_(n)where a is a parameter less than 1 but close to 1 and x_(n) representsthe string of states of the filter.
 3. Process according to claim 2,characterized in that it consists in allocating the value 256 to thefirst value of u_(n) and the value 0 to the second value of u_(n). 4.Process according to claim 2 characterized in that it consists inallocating the value {fraction (15/16)} to the parameter “a”.
 5. Processaccording to claim 2, characterized in that it consists in allocating avalue defined by the relation (2^(r)−2^(s))/2^(r) to the parameter “a”and the value 2^(q) to the first value of u_(n) and the value 0 to thesecond value of u_(n), in which q, r and s are positive integers. 6.Device for measuring the proportion of ATM cells satisfying a criterionat a specified point of a cell-based data transmission network,characterized in that it comprises a circuit (1) for detecting cellssatisfying a criterion U_(n) coupled with an exponential meancalculating circuit (2) of the string of values U_(n), composed of afirst-order digital filter defined by the equation X_(n+1)=ax_(n)+(1−a)U_(n) where a is a parameter less than 1 but close to 1 and X_(n)represents the string of states of the filter.
 7. Device according toclaim 6, characterized in that the exponential mean calculating circuit(2) comprises an addition operator (3) receiving the string of valuesu_(n) on a first operand input a first shift operator (4) which iscoupled to the output of the addition operator (3) so as to supply thestring of values x_(n), a second shift operator (5) coupled in serieswith a delay-by-one-bit circuit (6) between the output of the firstshift operator (4) supplying the string of values x_(n) and a secondinput of the addition operator (3), a third shift operator (7) coupledbetween the output of the delay-by-one-bit circuit (6) and a thirdoperand input of the addition operator (3) and a fourth shift operator(8) so as to apply the string of values u_(n) to the first operand inputof the addition operator (3).
 8. Device according to claim 7,characterized in that the first shift operator (4) performs a rightwardshift by r bits, the second shift operator (5) performs a leftward shiftby r bits, the third shift operator (7) and the fourth shift operator(8) perform a leftward shift by s bits.
 9. Device according to claim 7characterized in that it comprises shift operators having shift lengthsof r=4 and s=0 bits respectively.
 10. Process according to claim 3,characterized in that it consists in allocating the value {fraction(15/16)} to the parameter “a”.
 11. Device according to claim 8,characterized in that it comprises shift operators having shift lengthsof r=4 and s=0 bits respectively.